Metal halide arc discharge lamps are frequently employed in commercial usage because of their high luminous efficacy and long life. A typical metal halide arc discharge lamp includes a quartz or fused silica arc tube that is hermetically sealed within a borosilicate glass outer envelope. Recent advances in the art have employed a ceramic arc tube constructed, for example, from polycrystalline alumina. It is with the latter type that this invention is particularly concerned. The arc tube, itself hermetically sealed, has tungsten electrodes sealed into opposite ends and contains a fill material that may include mercury, metal halide additives and a rare gas to facilitate starting. In some cases, particularly in high wattage lamps, the outer envelope is filled with nitrogen or another inert gas at less than atmospheric pressure. In other cases, particularly in low wattage lamps, the outer envelope is evacuated.
It has been found desirable to provide metal halide arc discharge lamps with a shroud that comprises a generally tubular, light-transmissive member, such as quartz, that is able to withstand high operating temperatures. The arc tube and the shroud are coaxially mounted within the lamp envelope with the arc tube located within the shroud. Preferably, the shroud is a tube that is open at both ends.
The shroud or shield has several beneficial effects on lamp operation. In lamps with a gas-filled outer envelope, the shroud reduces convective heat losses from the arc tube and thereby improves the luminous output and the color temperature of the lamp. In lamps with an evacuated outer envelope, the shroud helps to equalize the temperature of the arc tube. In addition, the shroud effectively reduces sodium losses and improves the maintenance of phosphor efficiency in metal halide lamps having a phosphor coating on the inside surface of the outer envelope. Finally, the shroud improves the safety of the lamp by acting as a containment device in the event that the arc tube shatters.
In lamps using ceramic arc tubes, mounting the arc tube within a shroud has proven difficult and expensive. The ceramic arc tube has a tubular or bulbous body with ceramic, cylindrical capillaries extending therefrom. The capillaries are relatively small, often having diameters of 3 mm or so, and contain the electrodes. A number of techniques have been devised for solving the mounting problems of ceramic arc tubes as shown in U.S. Pat. Nos. 6,628,080; 6,741,034; 6,858,976; and 7,135,811, all of which are assigned to the assignee of the instant invention. Of these, U.S. Pat. No. 6,741,034 also teaches a starting aid in conjunction with a mounting clip and the teachings of the '034 are hereby incorporated by reference.
While the structures shown in the above-cited references have proven commercially successful, nevertheless, lamps so constructed have life spans shorter than anticipated. Often, this shortened lifetime is caused by cracks developing in the frit seal that fixes the electrode within the capillary.